In conventional mechanical structures the globally distributed stiffness and damping characteristics of the system are fixed parameters which cannot be easily changed or controlled once fabrication is complete. In order to limit the bending or flexing of the structural elements in an extended mechanical structure, the control of the stiffness and damping parameters are desirable. Applications in which controllable structural behavior will find utility include, but are not limited to, controlling vibrations and sound propagation in aerospace and automotive structures, such as walls and panels; flexible fixturing in manufacturing systems; and improving response time associated with the manipulation of robotic structural elements.
The result of the interaction between a mechanical structure and external dynamic forces, such as air/water-borne sound waves, direct excitation, and turbulences caused by the movement of the structure in air/water, is the generation and propagation of flexural waves within the structure. If the intensity of these resonant waves is large, the structure may become permanently deformed or damage to electronic or mechanical equipment attached to the structure may occur. The generation of waves in the structure of smaller intensity causes the radiation of unacceptable sound disturbances (i.e., noise), as well as vibrations that decrease the useful life of sensitive equipment attached to the structure.
The damping control of systems in which motion results in the bending or flexing of structural elements can only be adequately accomplished through the utilization of a distributed or global methodology. Although localized damping utilizing discrete devices, such as the electrorheological and magnetorheological fluid containing dampers or mounts described in U.S. Pat. Nos. 3,207,269, 4,720,087, 4,733,758, 5,277,281, and 5,284,330 can be used as couplings between the element(s) to be isolated and the source of the mechanical disturbance, they are inherently inadequate in controlling the overall constitutive characteristics of a mechanical structure. Spatially discrete damping devices, such as actuators, dampers and mounts, are limited in the number of vibrational modes that can adequately be controlled. In order to provide overall control of the desired stiffness and damping characteristics in an extended structure utilizing such devices, an unacceptable number/variety of damping elements would be required.
Conventional distributed damping in a structure involves placing a viscoelastic solid or polymer in contact with the surface of the vibrating structure. The viscoelastic polymer sometimes is coated with a thin constraining barrier layer in order to improve performance by increasing the dissipation of energy from the system. The problem with using a conventional viscoelastic solid is that the damping of the system is optimized for a single temperature and frequency. The poor performance of conventional passive damping methodology is related to the infinite number of frequencies at which resonant waves can propagate through a structure.
U.S. Pat. No. 4,565,940 describes an attempt to provide optimum damping control in a conventional system through the use of piezoelectric ceramic and polymeric films as a constraining layer. In this system the damping effect of the constraining layer is controlled by varying the electric voltage applied to the film. Unfortunately, the brittleness and difficulty associated with manufacturing large, thin piezoelectric ceramic films precludes their use in commercial applications. Although piezoelectric polymers are readily available in flexible films, they are incapable of producing the forces necessary to adequately provide the desired level of damping in any practical system.
Another approach to providing distributed control of the damping characteristics of a structure is provided in U.S. Pat. No. 4,923,057. This patent describes the utilization of electrorheological fluids as a medium through which the control of the overall dynamic properties of a structure can be achieved. In this system the electrorheological fluid is contained in a region within the structure. The variability in the complex shear and compression/tension moduli exhibited by the electrorheological fluid as a function of applied electric field strength allows for the control of the stiffness and damping characteristics exhibited by a structure. Unfortunately, the moduli exhibited by electrorheological fluids are several orders of magnitude less than that exhibited by conventional viscoelastic solids or polymers.
Conventional damping of an extended structure using a viscoelastic solid, constrained layers or discrete, localized devices, such as actuators, dampers and mounts, inherently suffer from an inability to control the overall stiffness and damping characteristics exhibited by a structure over a broad temperature and excitation frequency range. The utilization of piezoelectric ceramic or polymeric films as the constraining layer do not meet conventional system needs by being either brittle and difficult to manufacture or incapable of producing the forces necessary to adequately provide the desired level of damping. Although electrorheological fluids can provide distributed control of the overall damping characteristics of an extended structure, the moduli exhibited by these materials are several orders of magnitude smaller than the moduli observed for common viscoelastic solids. A need, therefore, exists for the development of a material that exhibits complex shear and compression/tension moduli comparable in magnitude to conventional viscoelastic solids along with the controllable characteristics exhibited by electrorheological fluids.